Nowadays, a plurality of electronic devices such as mobile phones, tablet PC or in general embedded devices use a backup battery which, as known by a skilled man is a secondary power supply that provides power to electronic devices in the absence of main power supplies. In particular, the backup battery is used to prevent losing sensitive information when the device is no more supplied by a main power supply, such as, for instance, real time clock data.
Conventional batteries used for embodying backup batteries are based on Ni-Cad and Lithium-Ion.
Also conventionally, electronic products manufacturers perform production tests to ensure that any of the components of a product operate properly after their assembly, and this also applies to the particular backup battery included in the product.
The most common method for performing a production test is the visual method that carries out a visual test for detecting the presence of the product components. Such visual test can applied, either manually by a human person or automatically by using automated optical inspection.
In order to make the production tests more reliable and faster, the trend is to involve more and more automation in production tests and thus generalizes the development of automated optical inspection applied to the PCB (Printed Circuit Board) during the manufacturing process, e.g. by means of a camera capturing the PCB image from the side of the PCB where components are being assembled. The inspection machine scans the PCB image in order to detect if all components are present and well placed.
Such method has been used also for checking the presence of the backup battery. However, the visual method—be it manual or automatic—shows limitation since it is not appropriate to detect proper soldering of the components or, even, proper working of the latter.
In order to further increase the reliability on checking the functionality of a product component as well as to decrease the duration of the testing process, more and more manufacturers integrate within the internal circuits forming part of an electronic product a specific circuit for achieving what is called auto-test (self-test) capability.
Accordingly, such electronic products have a capability to auto-test the functionalities of their components by themselves. In that respect, if a backup battery is included in one electronic product that supports auto-test capability, its functionality can be tested automatically.
One known auto-testing technique for detecting the functionality of an electronic product component and particularly of a backup battery, is the performance of a voltage measurement, which is achieved by the internal and General Purpose Analog-to-Digital Converter (GPADC) which is generally present in recent power management unit (PMU).
Such conventional auto-test method is illustrated in FIG. 1, showing an electronic product or appliance 10 which includes a Power Management Unit 20 to which is connected one battery 30, e.g. a backup battery 30. The Power Management unit 20 further includes a detecting block 40 which senses the voltage of the battery, plus a battery charger 50 and a General Purpose ADC converter 60 having access to all analog voltages present in the circuit for the purpose of converting them into the digital representation forwarded to a (not known) processor.
Particularly, GPADC 60 is used for measuring the voltage of the backup battery 30 in order to check its presence . . . .
a) Lithium-Ion or Ni-Cad Backup Batteries
The auto-test is traditionally applied to the Lithium-Ion and the Ni-Cad battery since it allows to achieve very fast testing. Indeed, Lithium-Ion or Ni-Cad backup batteries are never fully discharged and they have a minimum voltage guaranteed by the backup battery manufacturer. Consequently, during the manufacture of an electronic product including a Lithium-Ion or Ni-Cad backup battery, the test of a fully operative Lithium-Ion battery can simply be based on the checking of the voltage generated by such battery without any preliminary need to charge the latter. More particularly, If the measured backup battery voltage is between expected voltage thresholds the auto-test succeeds and thus the backup battery is presumed to properly operate and to be well soldered. Otherwise the auto-test fails.
Therefore, auto-test process, when applied to Lithium-Ion and/or Ni-Cad backup batteries, can be achieved in less than 1 ms.
b) Electric Double-Layer Capacitors
The most recent of the above mentioned backup battery technologies is the electric double-layer capacitor, also designated as being a GoldCap backup battery. It includes an electrochemical capacitor that has an unusual high energy density compared to common capacitors. Also, its manufacture cost is less than Lithium-Ion and Ni-Cad backup batteries.
The problem lies on the fact that the known auto-test process described above, can not be achieved in less than 1 ms.
Indeed, as known by the skilled man, an electric double-layer capacitor, in contrast with the Lithium-Ion or Ni-Cad backup batteries, is always discharged and its voltage is 0V. Consequently, the checking of the presence of a voltage on such a backup battery can not directly and immediately be performed, as for the Lithium-ion battery, just after the soldering operation, since a preliminary charging operation of the battery is required.
Such compulsory charging might take at least several hundreds of milliseconds, what would significantly impact the time for manufacturing the product and, eventually, jeopardize the productivity.
Indeed, considering for instance the flowchart of FIG. 2, there is described the adaptation of the auto-test process—known to the Lithium-Ion—for the new electric double-layer capacitor:    Phase I: initialization of the auto-test software    Phase II: charging of the backup battery during a period sufficient for entailing a first increase of the voltage of the battery, e.g. at least 500 ms    Phase III: performance of voltage stabilization that lasts for ˜100 ms    Phase IV: stop charging and measure the backup battery voltage, for instance by means of the General Purpose ADC convertor.
At the end of phase IV, if the measured backup battery voltage shows to be between the expected voltage thresholds the auto-test succeeds and thus the backup battery (in that case the electric double-layer capacitor) is presumed to be well soldered and operative. Otherwise, the auto-test fails.
The completion of the known auto-test process, when applied to those GoldCap battery, has required not less than 600 ms, what is significantly higher than the period required for testing Lithium-Ion and Ni-Cad battery.
Therefore, the application of the conventional auto-test process of FIG. 2 is not appropriate for electric double-layer capacitors or GoldCap batteries since it requires to much time.
There is a wish for an alternative auto-test which is more adapted to the new type of fully discharged backup batteries and which additionally provides more accurate test results.